Various types of hearing prostheses may provide people having different types of hearing loss with the ability to perceive sound. Hearing loss may be conductive, sensorineural, or some combination of both conductive and sensorineural hearing loss. Conductive hearing loss typically results from a dysfunction in any of the mechanisms that ordinarily conduct sound waves through the outer ear, the eardrum, or the bones of the middle ear. Sensorineural hearing loss typically results from a dysfunction in the inner ear, including the cochlea, where sound vibrations are converted into neural signals, or any other part of the ear, auditory nerve, or brain that may process the neural signals.
People with some forms of conductive hearing loss may benefit from hearing prostheses, such as traditional hearing aids or other acoustic hearing prostheses. A traditional hearing aid typically includes a small microphone to detect sound, an amplifier to amplify certain portions of the detected sound, and a small speaker to transmit the amplified sound into the person's ear. Other acoustic hearing prostheses typically include a small microphone to detect sound, and a vibration mechanism to apply vibrations corresponding to the detected sound to a person's bone, thereby causing vibrations in the person's inner ear, thus bypassing the person's auditory canal and middle ear. Such acoustic hearing prostheses include bone conduction hearing devices, direct acoustic cochlear stimulation devices, and middle ear devices.
A bone conduction device typically utilizes a surgically-implanted mechanism to transmit sound via direct vibrations of an implant recipient's skull. An external component of the bone conduction device detects sound waves, which are converted into a series of electrical stimulation signals delivered to the implant recipient's skull bones via an electromechanical transducer (e.g., a mechanical actuator). By providing stimulation to the recipient's skull, the bone conduction device enables the recipient's middle ear and auditory canal to be bypassed, which is advantageous for recipients with medical conditions that affect the middle or outer ear. The vibrations of the recipient's skull bones cause fluid motion within the recipient's cochlea, thereby enabling the recipient to perceive sound based on the vibrations. Similarly, a direct acoustic cochlear stimulation device typically utilizes a surgically-implanted mechanism to transmit sound by directly moving the ossicular chain of the recipient, which causes fluid motion within the recipient's cochlea or directly moving the fluid within the recipient's cochlea. Other non-surgical vibration-based hearing aids may use similar vibration mechanisms to transmit sound via direct vibration of a recipient's teeth or other cranial or facial bones.
Each type of hearing prosthesis has an associated sound processor. In one basic embodiment, the sound processor provides amplification to any sounds received by the prosthesis. However, in other embodiments, the processor present in a hearing prosthesis may be more advanced. For example, some processors are programmable and include advanced signal processing functions (e.g., noise reduction functions).
Sound processing systems may unintentionally introduce feedback into the audio system. For example, in a generic sense, in a sound processing system with a microphone and speakers, sound captured by the microphone may be amplified and output by the speakers. However, a portion of the sound captured by the microphone may include the sound produced by the speakers. When the microphone captures the sound produced by the speakers, which is an amplification of the microphone signal, undesirable acoustic or audio feedback may be produced.